In an engine for an automobile and the like, the blow-by gas leaking out into a crank case through a gap between a piston and a cylinder includes a large amount of hydrocarbon (HC). Since the hydrocarbon is a causative material of photochemical smog, a recirculation method is widely used, which returns the blow-by gas to an inlet system to burn again with an air-fuel mixture without emitting the blow-by gas to the atmosphere. Since the blow-by gas includes the oil wherein an engine oil and the like are pulverized, the gas after the oil is removed is sent to the inlet system. As for a device for separating an oil mist inside the blow-by gas, various types (an inertial collision type, a labyrinth type, a cyclone type, and the like) of oil separators are known.
In Patent Document 1, there is described an inertial-collision-type oil separator. In the oil separator, there is attached a leaf spring closing an opening portion of a flow passage in such a way as to be supported in a cantilever manner. The leaf spring is elastically deformed so as to generate a gap between the opening portion and the leaf spring, and the blow-by gas accelerates when it passes through the gap thereof. After that, the accelerated blow-by gas collides against a wall provided on a downstream side, so that the oil mist contained in the blow-by gas is attached to the wall by an inertial action to be collected. In this system, the blow-by gas collides against the wall at a high speed, so that even if an oil particle has a relatively small diameter, the oil separator can collect the oil particle.